The invention relates to magnetorheological compositions that have improved performance when exposed to magnetic fields. More specifically, the present invention relates to large particle magnetorheological compositions which have improved controllability.
Magnetorheological fluids are magnetic field responsive fluids containing a field polarizable particle component and a liquid carrier component. Magnetorheological fluids are useful in devices or systems for controlling vibration and/or noise. Magnetorheological fluids have been proposed for controlling damping in various devices, such as dampers, shock absorbers, and elastomeric mounts. They have also been proposed for use in controlling pressure and/or torque in brakes, clutches, and valves. Magnetorheological fluids are considered superior to electrorheological fluids in many applications because they exhibit higher yield strengths and can create greater damping forces.
The particle component compositions typically include micron-sized magnetic-responsive particles. In the presence of a magnetic field, the magnetic-responsive particles become polarized and are thereby organized into chains of particles or particle fibrils. The particle chains increase the apparent viscosity (flow resistance) of the fluid, resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid.
Many of the magnetic-responsive particles in the magnetorheological fluids are comprised of spherical ferromagnetic or paramagnetic particles typically 1 to 10 microns in diameter, dispersed within a carrier fluid. Small magnetic particle size permits easy suspension and the design of devices having small gaps. However, there are a number of disadvantages to using small size particles. For example, there is an insufficient supply of fine magnetic-responsive particles for applications in which magnetorheological technology may apply. Moreover, the use of fine particle iron limits the range of metallurgy that can be used due to the process used to obtain such particles. Carbonyl iron, the most commonly used iron, is derived from iron pentacarbonyl salts. The particles are xe2x80x9cgrownxe2x80x9d by precipitation, resulting in a spherical unreduced particle with a very low carbon content. Alternatively, if large particles could be used instead of small particles, blends of various metals could be made and then reduced in size by particle reduction methods. Further, small metal powders may be difficult to process since they can become dust explosion hazards when they approach a micron in size. Additionally, small diameter magnetic-responsive particles are much more expensive than larger particles.
According to Levin et al., xe2x80x9cSome Features of the Magnetorheological Effect,xe2x80x9d J. Engin. Physics and Thermophysics, 70(5):769-772 (1997), the most widely used and cheap powders of carbonyl iron contain spherical particles sized to microns. Levin et al. investigated the rheological properties of magnetorheological suspensions in a wide concentration range of particles of the disperse ferromagnetic phase in the presence of a magnetic field. The study summary stated that the range of control of the viscous stress increment in a magnetorheological suspension can be broadened by changing the size and shape of the ferromagnetic particles, introducing nonmagnetic particles into the dispersion medium and by heating the medium to the Curie temperature.
A need exists in the art for a magnetorheological composition which utilizes inexpensive large size, non-spherical magnetic-responsive particles and exhibits excellent magnetorheological properties when used in a magnetorheological fluid. This invention provides such a composition.
The magnetorheological compositions according to the present invention comprise magnetic-responsive particles having an average number diameter distribution of about 6 to about 100 microns and at least one additive that reduces the interparticle friction between the magnetic-responsive particles. The additive may be an inorganic molybdenum compound, a fluorocarbon polymer or mixtures thereof. In one embodiment, the magnetic-responsive particles are about 60 to about 90 weight percent of the total magnetorheological composition. In a further preferred embodiment, the magnetic-responsive particles are irregular or non-spherical in shape.
The invention also is directed to a magnetorheological fluid comprising non-spherical magnetic-responsive particles having an average number diameter distribution of about 6 to about 100 microns, a carrier fluid and at least one additive that reduces the interparticle friction between the magnetic-responsive particles. The invention is further directed to a magnetorheological fluid comprising non-spherical magnetic-responsive particles produced by water atomization, at least one additive that reduces the interparticle friction between the magnetic-responsive particles, and a carrier fluid.